This invention is related to a microstrip bandpass filter, and in particular to a compact microstrip bandpass filter with multipath source-load coupling.
In modern microwave communication systems, like satellite and mobile communication systems, compact microwave bandpass filters with low passband insertion loss and high stopband rejection are required. Due to current processing technologies of integrated circuits, bandpass filters based on planar techniques, like microstrip bandpass filters, are most commonly used in practical applications. Bandpass filters consist of planar resonators, such as split ring, miniaturized hairpin, stepped-impedance and parallel-coupled resonators, have been proposed for either performance improvement or size reduction. However, most of the applied bandpass filters face a tradeoff between low passband insertion loss and high stopband rejection. As a result, most of them have a passband insertion loss of over −2 dB when reaching a relatively high stopband rejection, like −30 dB, and conversely, have a low stopband rejection when approaching a smaller passband insertion loss.
Moreover, due to the rapid growth of the spectrum occupation and the growing demand for higher receiver sensitivity, bandpass filters with a wider upper or lower stopband in the adjacent frequency band are required to reduce interference between signal channels, which introduce an additional challenge for the design of high-performance bandpass filters. According to early researches, bandpass filters with couplings between the input and output terminals provide a number of alternative paths which a signal may take. Depending on the phasing of the signals, plural transmission poles in the stopband are achievable through multipath effect, which can be used in the optimization of exhibiting ripples in both passband and stopband.